Osmotically driven membrane processes, such as forward osmosis (FO) and pressure-retarded osmosis (PRO), rely on large osmotic pressure differentials across semi-permeable membranes to generate water flux, while retaining solutes on either side of the semi-permeable membrane.
Polymeric membranes currently used in liquid separations are thin-film composite (TFC) membranes. TFC membranes are typically comprised of a selective barrier, which sits on top of a porous structure called the support layer or porous support. In typical pressure-driven membrane processes, the driving force for water flux is the hydraulic pressure applied across the membrane. The water permeability of the selective barrier is much greater than the barrier's permeability to the solute; hence, the solute is rejected by the membrane. The porous support merely provides mechanical and structural support for the selective layer in pressure driven membrane separation processes, and plays no apparent role in the actual separation process.
Osmotically driven membrane processes make use of the osmotic pressure difference created by a concentrated draw solution to drive water across a selective barrier (e.g., a semi-permeable thin polymeric film). To achieve ideal performance in osmotically driven membrane processes, certain membrane characteristics need to be customized (see International Patent Application No. PCT/US08/005696, hereby incorporated by reference in its entirety). There exists a need for membranes, which are customized to optimize osmotically driven membrane processes.